Following the recent terrorist attacks, the agency began the largest investigation in its history and lawmakers pushed through a $40 billion emergency bill in part to fund efforts to recover from the attack and investigate it. But privacy groups have voiced concern that the bill might give the FBI the authority to use Carnivore in violation of constitutional privacy protections.

'The amendments are ambiguous, and that's a big problem,' said Jim Dempsey, deputy director of the Center for Democracy and Technology, a nonprofit public policy organization.

By adding a broad set of terms such as 'addresses' and 'routing information' to the amendment, lawmakers broadened the category of information that can be collected by Carnivore, pen registers and trap-and-trace devices, Dempsey said.

Pen registers are surveillance devices that capture phone numbers dialed on outgoing calls. Trap-and-trace devices record the original number from which a call is made.

The devices are not supposed to reveal the content of the communications, identify the parties involved or whether the call went through.

During congressional debate on the amendment, Sen. Patrick Leahy (D-Vt.) said the legislation gives law enforcement officials 'new abilities to go into people's computers.'

'Maybe the Senate wants to just go ahead and adopt new abilities to wiretap our citizens,' he said. 'We are going to do it with no hearings, no debate.'

But Sen. Orrin Hatch (R-Utah), who introduced the amendment, argued that the step was necessary. 'Federal investigators are often hampered when investigating terrorist incidents,' he said.

The FBI has been using Carnivore, which it renamed DCS 1000, for two years. On receiving court authorization, the bureau uses the system to tap into Internet communications, identify e-mail writers and record the contents of messages.

Dempsey said he is worried that the FBI could use the tool to conduct blanket screenings of e-mail.

'My concern with Carnivore is that it allows the government to monitor communication of many people, when they are looking for a few,' he said. 'Carnivore is controlled totally by the government, with no checks and balances.'

He said Internet service providers, which would act as buffers, should control the system, rather than the government.

David L. Sobel, general counsel for the Electronic Privacy Information Center, another nonprofit group, agreed.

'Carnivore was and is a very radical departure from the way things are done in this country,' he said. He said the amendment was a hastily made decision and cut short the debate on a complicated issue.